Computer networks

ABSTRACT

Nodes of a computer network are found by performing, in response to input of a label, a first retrieval operation ( 32 ) to identify the address of a node matching that label, and performing, in response to an address identified by the first retrieval means, a second retrieval operation ( 35 ) to identify the addresses of further nodes matching the same label. Each node matching a given label has a data storage area for containing the addresses of other nodes matching the same label and is responsive to inquiry messages to return a message containing the addresses of the list. The second retrieval operation includes sending an inquiry message to the address identified by the first retrieval means and upon receipt of a response iteratively sending further inquiry messages to addresses contained in the response to the previous inquiry message.

This application is the US national phase of international application

PCT/GB2004/003941 filed 13 Sep. 2004 which designated the U.S. andclaims benefit of GB 03022494.6, dated 25 Sep. 2003, the entire contentof which is hereby incorporated by reference.

BACKGROUND Technical Field

The present invention relates to computer networks, and particularly,though not exclusively to information retrieval in the context ofdistributed systems such as peer-to-peer systems, especially those withno centralised storage or control.

BRIEF SUMMARY

According to one aspect of the invention, there is provided a computernetwork containing nodes, and having

-   -   first retrieval means responsive to input of a label to identify        the address of a node matching that label;    -   second retrieval means connected to receive an address        identified by the first retrieval means and operable in response        thereto to identify the addresses of further nodes matching the        same label;        wherein each node matching a given label has associated with it        a data storage area for containing the addresses of other nodes        matching the same label and is responsive to enquiry messages to        return a message containing the addresses of the list;        and wherein the second retrieval means is operable to send an        enquiry message to the address identified by the first retrieval        means and upon receipt of a response to iteratively send enquiry        messages to addresses contained in the response to that enquiry        message or as the case may be in a response to a subsequent        enquiry message.

In another aspect, the invention provides a method of operating acomputer network containing nodes, comprising

-   -   performing, in response to input of a label, a first retrieval        operation to identify the address of a node matching that label;    -   performing, in response to an address identified by the first        retrieval means, a second retrieval operation to identify the        addresses of further nodes matching the same label;        wherein each node matching a given label has associated with it        a data storage area for containing the addresses of other nodes        matching the same label and is responsive to enquiry messages to        return a message containing the addresses of the list;        and wherein the second retrieval operation includes sending an        enquiry message to the address identified by the first retrieval        means and upon receipt of a response iteratively sending enquiry        messages to addresses contained in the response to that enquiry        message or as the case may be in a response to a subsequent        enquiry message.

Other, preferred, features of the invention are defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a computer used in one embodiment of theinvention;

FIG. 1A is a flowchart showing a data retrieval operation using primaryand secondary virtual networks;

FIG. 2 is a schematic diagram illustration the management of linksbetween nodes of a computer network;

FIGS. 3 to 10 are flowcharts showing aspects of the operation of a nodeof the secondary virtual network;

FIGS. 11 to 14 and 16 are flowcharts showing aspects of the operation ofa node of the primary virtual network; and

FIG. 15 is a schematic diagram illustrating the flow of messages duringthe process depicted in FIG. 14.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS NODES

In this description reference will be made to computing nodes that haveprocessing, storage and communication capabilities. A computing node canbe a computer or other device, or—noting that a single computer may havea number of independent programs or processes running on it—may be asuch a program or process. An item of stored data may also be regardedas a distinct node, even though a number of such items may be servicedby a single program or process.

This description assumes that each computing node is connected to somecommunication infrastructure which could for example be atelecommunications network such as an IP (internet protocol) network, sothat messages can be sent to it. Thus, each computing node alsoconstitutes a node within the communications infrastructure.

Reference will also be made to virtual nodes which belong to a virtualnetwork. The distinction is important because a computing node is ableto have two or more virtual nodes (possibly belonging to differentvirtual networks) associated with it. As its name implies, a virtualnode does not exist in any physical sense: rather, as will become clearpresently, its existence is established by stored data which definelinks between virtual nodes and, hence, also define the virtual networkto which it belongs.

Necessarily a virtual node must be associated with a computing node,which provides it with processing, storage and communicationcapabilities: references to the sending, receiving and processing ofmessages by a virtual node refer to such sending receiving or processingby the computing node on behalf of the virtual node.

An example is shown in FIG. 1. A computer has the usual components,namely a processor 1, memory 2, display 3, keyboard 4 and acommunications interface 5 for communication via a network 10.

The memory 2 contains operating system and other programs (not shown),and data files such as the text file 20 shown. It also has storage 21containing a label 21 a corresponding to the text file 20 and its ownaddress 21 b. In addition, it has an address list 22 and a supportingprogram 23 which together define the existence, on the computer, of anode of a virtual network. This node has an address 24. Also shown arean address list 25 and a supporting program 26 which together define theexistence, on the computer, of a node of another virtual network. Thisnode has an address 27. The addresses stored in the lists 22, 25 are theaddresses of other nodes in the same virtual network.

Look-Up System

We will now describe a distributed look-up system, though this is onlyone possible example of an application for the invention. This systemallows users to associate comments with a web page. Whenever a uservisits this page, he has the opportunity also to view the comments thatother users have made. The comment is stored on the computer of the userthat contributed the comment (e.g. as a text file).

The user viewing the web page (or rather, his computer) has theuniversal resource locator (URL) of the web page, and what is requiredis a mechanism whereby he can retrieve the comments. In this example themechanism is as follows:

The text file is stored on the computer of the user that contributed thecomment and is associated with a node of a virtual network of the typedescribed in our international patent application no. WO 03/034669[Agent's ref. A30044], as too may be other text files containingcomments about other web pages, and possibly other unrelated files too.This virtual network (referred to in the context of the presentdescription as the primary virtual network, or simply the primarynetwork) serves to allow one to send a message to a node without knowingits address provided one has a label which identifies it. Although thattype of network can function with unique labels (one per node), in thisexample the labels are not unique: rather, all nodes associated withtext files containing comments about a particular web page have the samelabel. This label is a hash function of the URL of the web page. Thisvirtual network offers a retrieval mechanism which reaches only onenode.

The text file is also associated with a node of a second virtualnetwork. This (the secondary virtual network) contains only nodesassociated with text files containing comments about the one particularweb page.

Note however that whilst the use of a primary network in accordance withour aforementioned international patent application is preferred, it isnot essential. Indeed, it is not essential to use a virtual network atall; another primary retrieval mechanism which receives a label andreturns the address of one node corresponding to it could be usedinstead.

The computer posting a comment is as shown in FIG. 1 and must

-   -   create a node in the primary network. This node has a label 21 a        and a network address 24.    -   create a node in the secondary network. This node has a network        address 27.

Initially the address lists 22, 25 are empty, except that the list 22contains bootstrap links. The self-organisation of the networks toensure that the list 22 contains the labels and addresses of some othernodes of the primary network and that the list 25 contains the addressesof some other nodes of the secondary network will be described later.For the time being, the system will be described on the assumption thatthese labels and addresses are present.

A few words about addresses are in order at this point. The node formedby the text file 20, the node of the primary virtual network and thenode of the secondary virtual network, whilst conceptually having asingle identity, have their own addresses. It would be possible toallocate to each node a distinct address within the communicationsnetwork 10, although in practice this is not particularly convenient. Inour preferred implementation each node has an address consisting ofthree parts:

-   -   An internet address, which “locates” the computing node. E.g.        130.146.209.15    -   A port number, which locates a particular communication port at        the computing node. Ports are a standard part of Internet        addresses. They for instance allow different independent        application programs to independently send and receive messages.        I.e. each would receive messages at its own port, and would not        receive or be “confused” by messages intended for other        application programs. The Internet address together with the        port number can be considered to be the network address (as it        is part of the communication protocols, such as TCP/IP, that are        used).

The network address for all primary and secondary nodes can be the same,however, not necessarily so. For instance, all messages for primarynodes may be received at a different port from that at which secondarymessages are received (which is one way to distinguish between suchmessages).

-   -   A node identifier (an integer value), which locates the specific        node for which the message is intended. e.g. if all messages on        the primary network are received at a dedicated port, there is        still a locally unique identifier associated with each node. So,        when there are multiple nodes, it is clear for which node the        message is intended. This node identifier is an        application-specific address extension (it's not part of the        standard Internet protocol). It is simply included in the        message that is sent. The process that receives it “knows” this        and will examine this node identifier to determine to which node        the message should be forwarded.

It is possible that both nodes have the same network address, but notnecessarily so. Not every node will have a port of its own (partlybecause the number of available ports is somewhat limited), but one maywell have two ports (and-thus two different network addresses): one forthe primary network and one for the secondary network. Typically, therewill be a number of secondary networks, which could all use the sameport.

It should be stressed that, in the following, references to the addressof a node refer to the complete address of that node.

A particularly attractive approach is to provide that a text file andthe primary and secondary nodes all have the same node identifier (andIP address), only the port numbers being different. Such an addressingprotocol may provide an opportunity for simplifying some of theprocessing in that, where one has the address of one node and requiresthe address of another node associated with it, the address of thelatter node might be deduced from that of the former, rather than haveto be looked up. In the following description, however, no such,simplification has been made, so that these processes will work with anyaddress protocol.

The computer viewing a web page retrieves the associated comments by

-   -   applying the same hash function to the URL to obtain the label    -   sending a query (containing the label) on the primary virtual        network, to obtain the address of one node    -   using the address found, sending a query on the second virtual        network to obtain the addresses of more (or even all) all other        nodes on the second virtual network.    -   using these addresses to retrieve the comments for display.

Note that the retrieving computer does not necessarily have to containnodes of the virtual networks; it can be a conventional computer loadedwith software for implementing the retrieval process, and with acommunication interface so that it can communicate with the computers onwhich the nodes of the virtual networks reside. This process is shown inthe flowchart of FIG. 1A, and proceeds as follows:

-   Step 30: following the user inputting a URL (or invoking a    hyperlink) the computer retrieves the corresponding web page. This    step is entirely conventional.-   Step 31: a hash function is applied to the URL to obtain a label. As    discussed in our earlier international patent application, this    could use the SHA-1 algorithm.-   Step 32: a ‘Find’ message, containing this label and the network    address of the retrieving computer, is sent to a node of the primary    network. Manifestly it is necessary for the computer to be in    possession of at least one such address.-   Step 33: the retrieving computer receives a ‘Found’ message from the    primary network. This message contains the label and address of a    node that has been found as well as the addresses of the associated    node of the secondary network, and of the comment. A timeout    mechanism could be included to abort the process if a Found message    is not received in a reasonable time.-   Step 34: in this example, the primary network is arranged so that it    always returns the label and address of the node having a label    closest to the label contained in the Find message. So a check is    performed to see if the label that is returned is the same as that    asked for, and if not, the process is terminated. See below for an    explanation of the meaning of “nearest”.-   Step 35: assuming that the labels match, the retrieving computer    executes a process (to be described in detail below) whereby it uses    the address returned by the Found message to retrieve further    addresses using the secondary network.-   Step 36: These addresses are then uses to retrieve from the    “posting” computers the text files containing the comments.    The Secondary Virtual Network.

The aim of this network is to self-organise a group of nodes into asingle virtual network, which can subsequently be used to discover allnodes that are part of the group. The main requirement is that theresulting network contains all nodes. Another requirement is that thesystem load that is needed to create and maintain the network is spreadequally across all nodes. Not only is this the most “fair”, which isimportant when different users contribute their resources to adistributed application, it also helps to protect the system againstoverload.

The network, therefore, has the following properties:

-   -   The number of links maintained by each node is preferably the        same.    -   All links are bi-directional. As a result, the number of links        to a node are also the same for each node. This is important, as        this affects the number of a messages that a node receives and        must handle.    -   It has a “flat” structure. The nodes do not arrange themselves        hierarchically. As a result, the system load is spread equally        across all nodes.        Structure of Each Node

Each node has the following data associated with it:

-   -   Several links to other nodes. Each link is simply the address of        another node. Associated with each link is a status, which can        be “confirmed” or “unconfirmed”. Each node can only maintain a        maximum number of links, which is given by the system wide        parameter L. A typical value for L is for instance 6. It is not        essential that this parameter be the same for all nodes; but        there is no advantage to be gained by making them different.    -   A list of spare links, or spares in short. Each spare is simply        the address of another node. The spares are used by the        self-organisation process to build the virtual network. A node        adds other nodes as spares when it is notified about a node that        it cannot add as a link, either because it already links to the        node, or because it has the maximum number of links already. The        number of spares that a node can maintain is also limited, and        given by the system wide parameter S. A typical value for S is        for instance 3. The list of spare links is not essential in        general, but is very valuable in providing an additional        mechanism whereby a link that cannot be accommodated locally can        be propagated to some other point in the virtual network.        However the use of spare links (or a similar propagation        mechanism) is necessary in systems where the incoming Notify        messages always arrive at the same node (or one of a very small        number of nodes) of the secondary network.        Messages

In order to self-organise into a network and to discover which nodes arepart of a given network, nodes send messages to one another: Thefollowing types of messages are used by the secondary network:

-   -   AddLink message with:        -   address of sender        -   address of receiver

It is sent by a node (sender) to another node (receiver) to request amutual link.

-   -   ChangeLink message with:        -   address of sender        -   address of receiver        -   address of subject

It is sent by a node (X) to another node (Y) to request that it changesone of its links (Z) to a link to itself (X). The protocol is such thatX will send a similar message to Z requesting it to change its link to Ywith a link to itself (X). So, effectively, X requests to insert itselfin the link currently between Y and Z.

-   -   LinkAdded message with:        -   address of sender        -   address of receiver

It is used to notify a node that the sender just added a link to it.

-   -   LinkError message with:        -   address of sender        -   address of receiver        -   address of subject        -   error code

It is used to notify a node that there appears to be a problem with oneof its links. For instance, the subject node may not respond, or thelink may not be mutual. It includes an error code to indicate the typeof error.

-   -   Links message with:        -   address of sender        -   address of receiver        -   addresses of all links        -   reference value        -   the Links message can also contain some other data from the            sender node. In the web page comment example this is the            address of the associated comment

It contains all the current links of the sending node. It is always sentin response to a LinksQuery message. The reference can be used todistinguish the specific query that is responded to.

-   -   LinksQuery message with:        -   address of sender        -   address of receiver        -   reference value

It is used to request a node to send a Links message in reply(containing its current links).

-   -   Notify message with:        -   address of sender        -   address of receiver        -   address of subject        -   notify level

It is used to notify a node of another node in the network. The notifylevel is used to control and limit the propagation of Notify messages.As described here, sender address is not used, but is useful fordebugging or if it is desired to send acknowledgements.

Building the Secondary Network

The system lets a group nodes self-organise into a single, virtualnetwork, so that if one has the address of one node one can find theaddresses of others in the group. This section describes how new linksare created when nodes that should belong to the same secondary networkare discovered. Two parts can be distinguished here:

Discovery of pairs of nodes that should belong in the same secondarynetwork. What the criterion is for grouping nodes into the same networkis application specific. In the web page annotation example, all nodesthat represent comments about the same URL should be grouped together ina secondary network. How nodes are discovered that should be groupedtogether is also application-specific. An example is given shortly.

Updating/extending the secondary network as a result of node discovery.When a pair of nodes is discovered that should belong to the samesecondary network, the system may build one or more new links as aresult. The new link is not necessarily between the pair of nodes, butmay for instance be between nodes that these two nodes link to. How newlinks are created is described in detail later.

Initial Notify Message

The organisation of the secondary network presupposes the existence ofincoming ‘Notify’ messages that may for example identify an existing anda new member of the group (although early on, it is possible thatneither node is yet part of the group, whilst, later in theself-organisation process, both nodes might already be part of thegroup). It is up to another part of the system to notify the secondarynetwork of nodes that should belong to it. There are different ways inwhich it can be done. Here we give an example of how this is done whenthe secondary network is used in combination with a primary network ofthe type described in our earlier international patent application. Inthe web page annotation example, each comment publishes itself as a nodein the primary network under a label based on the URL of thecorresponding web page. This way, the primary network can be used tolook-up a comment for a given URL, if one exists. In order to show allcomments for a given URL, each comment also has a node of the secondarynetwork associated with it. Nodes that correspond to comments about thesame URL self-organise into a secondary network specific to that URL.This way, once the primary network is used to find a single commentabout a URL, the secondary network can be used to find other commentsabout the same URL.

So in this case, nodes of the secondary network that should be groupedtogether are each published under the same label in the primary network.A mechanism whereby in the primary network, nodes periodically execute a‘Push’ update to build and maintain links will be described below,including a modification so that whenever a node becomes aware ofanother node published under the same label, the needed Notify messageis generated.

Handling Notify Messages

When a node receives a Notify message about a node that it does not yetlink to, one of the following will happen:

If the receiving node already has the maximum number of allowed links,it adds it as a spare instead (unless it already had it as a spare). Ifin doing so, the node would exceed its maximum number of spares, itremoves one spare. It may then also forward the Notify message to thespare it removed. Whether or not it does so depends on the value of thenotify level. The notify level is decreased each time to preventmessages from propagating endlessly.

Otherwise, if the subject node does not yet have the maximum number oflinks either, the receiving node attempts to create a mutual linkbetween both nodes. This is illustrated in FIG. 2, diagrams a and b.Here, L=3 and Node 1 has received a Notify message about Node 2. Becauseboth nodes only had two links, a link is created between Node 1 and Node2.

Otherwise, when the subject node already has the maximum number oflinks, it is not possible to simply create a mutual link between bothnodes. So what happens is that receiving node attempts to insert itselfin an existing link. This is illustrated in FIG. 2, diagrams c and d.Here, the link between Node 2 and Node 3 is broken, but it is replacedby two new links: a link between Node 1 and Node 2 and a link betweenNode 1 and Node 3. So the total number of links is increased by one. Itworks even though Node 2 and Node 3 already had the maximum number oflinks. However, Node 1 needed to be able to create two new links forthis to succeed. The process is explained in more detail in theflowcharts of FIG. 3 to FIG. 9.

FIG. 3 shows how a node handles incoming Notify messages. Here it isdecided whether a new link should be created, and if so how (by adding anew link or by changing an existing link into two links). If no newlinks are created, the set of spares may be updated and another Notifymessage may be sent.

At Step 300, a Notify message is received, containing the address of thenode that sent it (sender), the address of the subject node, and apropagation limit value, notifylevel. The receiving node firstly checks(301) whether it has space to set up a new link and if so, whether (302)it already has a link to the subject node. If not, it attempts to set upa link with subject.

In Step 303 it sends a LinksQuery message to the subject node, and at304, awaits a reply. Once the reply—a Links message—is received, itagain checks (305) whether it still has space to set up a new link (incase it has received and handled any other messages in the meantime andcreated links as a result). If so, it then (306) examines the receivedLinks message to check whether the subject node has the space to set upa new link. If it has then at Step 307 and 308 the receiving node addsthe address of the subject node to its list of links (but marked“unconfirmed”) and sends an AddLink message to the subject node.

If however at Step 306 it is determined that the subject node cannotaccept further links, the receiving node then attempts to insert itselfinto an existing link as mentioned earlier with reference to FIG. 2. Thefirst step (309) is to check whether the receiving node has space fortwo links; if not, the process is terminated. If however it has, thenthe receiving node selects a link at random from the list of links inthe received Links message (but not a node to which the receiving nodealready has a link), that is, a link between the subject node andanother node referred to here as other. The receiving node then attemptsto insert itself into this link by:

-   311 adding the address of the subject node (unconfirmed) to its list    of links;-   312 adding the address of the other node (unconfirmed) to its list    of links;-   313 sending to the subject node a ChangeLink message containing the    address of other;-   314 sending to the other node a ChangeLink message containing the    address of subject.

Supposing however that at Step 301 it is determined that the receivingnode has no space to add a link, or that at Step 302 it already has alink to the subject node, then the process examines whether thereceiving node should add a link to its list of spare links. In Step 315the process terminates if it is found that the subject node is alreadyin the spares list. At 316 it is checked whether there is space to add alink to the spares list and if so this is duly added at 317. If not,then an existing one of the spare links is selected at random at 318,and removed at Step 319 so that it may be replaced by a link to subjectat Step 317. Also, the variable notifylevel is decremented at 320 and if(Step 321) it remains nonzero the original Notify message—with this newvalue of notifylevel—is forwarded at Step 322 to the node (referenced asreplace) pointed to by the randomly selected existing link.

The effect of this process is that when a node A that already has a fullset of links receives a Notify message asking it to link to a subjectnode B, B's address is recorded as a spare link. This link remainsdormant until A's list of spare links is full. Then, when A receives alater Notify message asking it to link to node C, and the spare link tonode B is selected at Step 318, the new Notify message generated at Step322 is in effect a request to node B to create a link from itself tonode C.

A mechanism is also provided—but not shown on the flowchart—whereby whena link is unconfirmed and the receiving node does not receiveconfirmation (by way of a LinkAdded message as described below withreference to FIG. 6) within a give period of time, the unconfirmed linkis deleted Note that when the receiving node has links that still havean “unconfirmed” status, it returns these unconfirmed links (as well as,of course, the confirmed ones) in response to LinksQuery messages,allowing other nodes to confirm that it is attempting to set up thelink.

In FIG. 3, the “no” exits of Steps 305 and 309 lead to termination ofthe process: however if desired they could be routed to the “spare link”process commencing at Step 315, with a slight improvement in efficiency.

In Steps 309 to 314, the node effectively breaks one of subject's linksand inserts itself in between. Another possible option, not shown in theflowchart, would be for the node to break one of its own links (assumingof course that it already has at least one link) and insert subject inbetween. This option, if implemented, would be tried immediately afterthe ‘no’ exit from Step 301. Firstly the receiving node would need tocheck whether subject had fewer than L−1 links, select at random one ofits own links (to a node other), replace this with an unconfirmed linkto subject, and send an AddLink messages to subject. In order toestablish a bidirectional link between subject and other it would then(a) send to subject a special AddLink message requiring subject to add,unconditionally, other as an unconfirmed link to its list of links and(b) send to other a special ChangeLink message with the receiving nodeas the old link to be removed and naming subject as the new link to beadded. This option could be included as well as, or instead of, Steps309 to 314.

Another option for the receiving node to break one of its own linkswould be for it (having firstly verified that subject had fewer than L−1links) to send to subject a Notify message naming itself as subject.This would have the same outcome but involve a slightly larger messagingoverhead.

FIG. 4 shows how a node handles incoming ChangeLink messages. Thesemessages are sent when a node X that received a Notify message wants tochange an existing link into two new ones (see FIG. 2). The receivingnode Y receives at 400 a Notify message with node Z as subject, i.e.asking node Y to replace its existing link to node Z with one to node X.If it already has a link to X, it takes no further action (401), whilstif (402) it does not in fact possess a link to node Z it sends 403 anerror message to the sender, X.

Assuming all is well, it sends (404) a LinksQuery message to the senderX and awaits (405) a Links message in reply from the sending node X tocheck that the latter has indeed created the two new links it shouldhave created before changing the subject link. If these checks (406,407) are successful, The receiving node removes its link to Z (408),adds X as a confirmed link (409) and returns a LinkAdded message to thesender X (410).

FIG. 5 shows how a node handles incoming AddLink messages. Thesemessages are sent when a node wants to create a new link with a node(see FIG. 1). The message having been received at 501, the node checksat Step 502 whether it has space for another link and if not, returns anerror message at 503. Otherwise, it sends (504) a LinksQuery message tothe sender and awaits (505) a Links message in reply from the sendingnode, so that it may check at 506 that the latter has indeed created alinks to the receiving node. If no, it declines to add the link andterminates, but if so it then adds sender as a confirmed link (507) andreturns a LinkAdded message to the sender (508) by way of confirmation.

FIG. 6 shows how a node handles incoming LinkAdded messages. Thesemessages are sent when another node has accepted a link to the receivingnode, either in response to a ChangeLink or a AddLink message. When theLinkAdded message is received at 600 indicating that a link has beenaccepted, its status is changed to “confirmed” at Step 601. The linkwill then be maintained until either it is changed for a new link (inresponse to a ChangeLink message), or the link is broken.

FIG. 7 shows how a node handles incoming LinkError messages. Thesemessages are sent when either a node was unable to create a link to thereceiving node after the latter requested a mutual link (by way of aChangeLink or AddLink message), or a link appears to be broken (the nodeat the other end may not be responding to messages, or the link may notbe mutual). Broken links are not detected by the self-organisationprocess, but when clients traverse the secondary network (as will beexplained later).

Following receipt of the message at 700 it is determined (701) whetherthe message is about a node to which the receiving node has anunconfirmed link. If so, and (702) it carries an error code indicatingfailure to created a requested link, then the link is removed at 703. Ifhowever the message is not about a node to which the receiving node hasan unconfirmed link, the receiving node sends (704) a LinksQuery messageto the subject, awaits (705) a Links message in reply, checks the replyat 706 to check whether the subject has a link to itself, and if notthen in Step 703 removes its link to the subject node.

FIG. 8 shows how a node handles incoming LinksQuery messages. Thesemessages are sent when another node wants to know the links of thereceiving node, and the latter upon receipt thereof at 800 thereforeresponds at 801 with a Links message.

FIG. 9 shows how a node handles incoming Links messages. How it ishandled depends entirely on why the corresponding LinksQuery message wassent. This happens for different reasons, as shown amongst others inFIG. 3, FIG. 4, FIG. 5 and FIG. 7. So what happens is that when aLinksQuery message is sent, it is given a reference that is locallyunique and a message handler is associated with the reference. Then,when a Links message is received (900), the appropriate message handleris identified and the message is forwarded at Step 902 to theappropriate message handler so that the message it dealt with in theright way.

It may of course happen that no Links message is ever received inresponse to a LinksQuery, for instance because the receiving node hasbeen shut down. Therefore, if after a given period no Links message hasbeen received the corresponding message handler is removed. Althoughthis has not been explicitly shown in any of the flow charts discussedhere, it simply means that when a links query times out, no furtheraction is taken and the entire flow chart is “done”.

Retrieving Nodes

Given the address of a single node of the secondary network, it ispossible to discover other, potentially all, nodes in the network. Theway that this can be done is very simple. One sends to the known node aLinksQuery message to request all its links. The node replies with aLinks message, containing the address of all the nodes it links to. Onecan then contact each of these nodes in turn, requesting their links andthus obtain the addresses of all their links. By continuing in this way,one traverses the network and gradually discovers all the nodes itcontains.

FIG. 10 shows the process in more detail. It will be understood thatthis is the process used in the retrieval step 35 shown in FIG. 1A. Theaddresses of all known nodes that have successfully been contacted areput in the “confirmed” list. Data may be retrieved at the same time. Inthe case of the “web page comment” example, the relevant item of data isthe address of the comment, and this too is entered into the confirmedlist alongside the node address. The confirmed list then provides theaddresses needed for the “Retrieve” comments step (36) in FIG. 1A. The“unconfirmed” list, on the other hand, contains the addresses of knownnodes that have not yet been contacted. Finally, the “known” listcontains the addresses of all known nodes. It includes all addresses inthe “confirmed” and “unconfirmed” list, but also the addresses of nodesthat have been contacted and that have not responded. The known listalso has, for each address entered into it, an additional field forcontaining a source address—that is, the address of the node from whoselist the address to which the current pointer points was obtained, forerror reporting purposes.

It is not material where the retrieval process occurs: it may be at anode, or somewhere else. At Step 1000, a request to retrieve nodeaddresses is received along with a start address, that is, the addressof one node that had been determined to belong to the virtual network inquestion. In Step 1002, an address pointer, current, is initially set tothis address whilst a second address pointer, source is initially null(1003).

At Steps 1004 and 1005 a LinksQuery message is sent to the address givenby current, and a reply awaited. When a Links message is received,current is added to the confirmed list (Step 1006), with the commentaddress from the Links message alongside it.

At step 1007, a sub-process is entered, which is performed for each ofthe addresses contained in the Links message. If (1008) the address isalready in the known list, the process steps on to the next address.Otherwise the address is added to the known list and to the unconfirmedlist (Steps 1009, 1010). Also (1011), the address in current is enteredinto the known list as being the source of the address added.

Once this sub-process is complete, then (unless the unconfirmed list isempty, in which case the process terminates at Step 1012) at Step 1013an address is selected at random from the unconfirmed list. This addressbecomes the new current address, and is deleted from the unconfirmedlist. The next step (1014) is to look up current in the known list toretrieve the source address associated with it, and enter this in thesource pointer. The random selection is not mandatory. E.g. currentcould be chosen to be the “oldest” node in the unconfirmed list, or thelist could be sorted by another criterion (e.g. node's addresses) andcurrent could always be the “first” node in this list. However, randomchoice of current has its advantages. It spreads the load in the system(in particular if not all nodes are always retrieved), and also spreadsthe testing of the links of the network so that broken links arediscovered more quickly.

The process then continues again from Step 1004 and iterates until theunconfirmed list is empty—i.e. no further new addresses can be found.

A side effect of the retrieval process is that it discovers brokenlinks. For instance, it may happen that a node is not responding, orthat a link is not mutual. The latter is the case when a node A links tonode B, but node B does not have node A in its link table. When a brokenlink is discovered, the node that is the “source” of the link isnotified by way of a LinkError message. As FIG. 7 already showed, thesource node can then check the link itself (to confirm the accuracy ofthe error report) and may remove the link as a result. A node that isnot responding is recognised by the failure at Step 1005 to receive aLinks message within a set time-out period and at Step 1015 an errormessage, containing the address of current and a “no reply” error code,is sent to source, whereupon control returns to Step 1012. Thenon-mutuality of a link is recognised by testing at Step 1016 todetermine whether the Links message received for current contains theaddress of source: if not, an error message, containing the address ofcurrent and a “not mutual” error code, is sent (Step 1017) to source,but the retrieval process continues as before, as it is theresponsibility of the source node to take remedial action (in accordancewith the process of FIG. 7). The test at Step 1016 is skipped if sourceis null.

Note that even though multiple confirmed nodes may link to a node thatdoes not respond to a Links message, only the node that firstcontributed the link (the source node) is notified that there was “noreply”. This is partly because it makes the flowchart easier tounderstand. However, it can be argued that there is another, practicalbenefit. It may be a case that a node does not reply (in time) becauseit is temporarily overloaded. In this case, one may not want multiplenodes to simultaneously sent it a LinksQuery message to test if there isan error (as in FIG. 7). Either way, if desired, it is straightforwardto update the node retrieval algorithm to notify all known nodes thatare affected by a broken link, when such a link is discovered.

In FIG. 10 the node retrieval does not stop until all known nodes havebeen contacted. In practice, one may wish to terminate the processearlier. For instance, if a user is looking for a location from which todownload a file, it may be sufficient to offer him or her the choice often potential download addresses instead of, say, all thousand.

The algorithm in FIG. 10 is shown as entirely serial. Only one node iscontacted at a time. Another LinksQuery message is sent only after areply has been received to the previous one (or it has been timed out).In practice, however we prefer to speed up the retrieval by issuingmultiple LinksQuery messages in parallel. It may also be the case thatat box 1000 multiple retrieval requests are simultaneously handled bymultiple instances of the process of FIG. 10.

Discussion

Successfulness of Self-organisation

The aim of the secondary virtual network is to self-organise all nodesthat should be grouped together into a single network, as opposed toseveral unconnected networks. Whether or not this is the case dependslargely on how the initial Notify message is generated. For instance, ifthere is a group of twelve nodes that should all be grouped together,but of this group five nodes only receive notifications about othernodes in this group of five, and none of the other seven nodes arenotified about any of these five nodes, it is impossible for the nodesto self-organise into a single network. Instead, they arrange into twoseparate networks, one of five nodes, and one of seven nodes. However,as long as the initial notifications are not such that it is impossiblefor nodes to self-organise into a single network, the self-organisationprocess is such that it is very unlikely that nodes do not self-organiseinto a single network. Calculation of the probability that theself-organisation results in a single network is complicated and dependson the mechanism by which the initial notifications are generated.However, in simulations we have experimented with several differentinitial notification mechanisms, and so far nodes never failed toself-organise into a single network.

Robustness to Malicious Nodes

So far it has been assumed that all nodes obey the protocol. However, itis possible that there are malicious nodes that do not play by therules. They may try to break links maintained by other nodes and/or tryto obtain too many links to themselves. It is desirable that the overallsystem is as robust as possible to such abuse.

The system described so far is already fairly robust to malicious nodes.That is because each node always check with a LinksQuery-Links messageexchange the links maintained by the other relevant node before changingits own links. For instance, when a node receives an AddLink message(see FIG. 3), it first checks that the sending node has indeed linked toit, before adding the sender as its own link.

However, the system still has a relative weakness. As it stands, nodescan easily “lie” when they respond with a Links message. Often a nodesends a LinksQuery message to check that the receiving node links to it.Knowing this, the receiving node can reply with a faked Links messagemodified such that it always contains the sender of the Links message asa link. This enables a node to have much more than the allowed number ofL nodes linking to it. This would, consequently, reduce the overallnumber of “good” links in the system.

Fortunately, there is a way to address this weakness. This can be doneif nodes sent their LinksQuery through a proxy node. These proxies arerandomly chosen each time a node want to send a query. Each node can forinstance use the nodes it currently links to as proxies. This way, thenode (A) that wants to know the links of another node (B) is unknown toNode B, because the LinksQuery messages it receives is from a proxy node(C), and the message that Node B receives from Node C does not refer toNode A at all. Therefore there is no good way for Node B to send fakemessages that have a significant effect on the overall system.

Of course, there's the question of what the effect is of maliciousproxies. Although obviously malicious proxies have a detrimental effect(it is inevitable that nodes that do not obey the protocol affect theperformance to some extend), this effect is limited. The reason is thatthey can only maliciously handle the LinksQuery that they are asked toforward, and these requests are spread roughly equally across all nodes.On the other hand, when proxies are not used, malicious nodes can causehavoc by being very active. If these nodes send many spurious AddLinkmessages, and fake the many Links message they subsequently send, theeffect on the overall system is much larger.

Primary Virtual Network

The primary network is described in detail in our aforementionedinternational patent application. Here, the basic retrieval andself-organisation mechanisms will be described, along with amodification that enables the generation of Notify messages for drivingthe self-organisation of the secondary network.

Firstly it is necessary to explain the concept of virtual coordinatespace used by this mechanism. It has already been mentioned that eachnode has a label. The label is translated into coordinates in a virtualspace. The space can be one, two, or higher dimensional. The precisetranslation mechanism is not very critical: for a one-dimensional spacethe label, considered as a binary number, can be used directly as thecoordinate. For two or more dimensions the preferred method is that thelabel, considered as a string of bits, is partitioned into two or moreequal groups, each group, considered as a binary number, forming one ofthe coordinates. Each coordinate (or the coordinate, in aone-dimensional space) is scaled to lie in the range [0,1].

The distance between two labels in this virtual space is the Euclideandistance between the two coordinate sets (though other distances such asthe city block distance (often called the Manhattan distance) could beused if desired. The coordinate space wraps, so that the distance in thex-direction between x₁ and x₂ isMin {(1−|x₁−x₂|),|x₁−x₂|}and the Euclidean distance in two dimensions between points (x₁,y₁) and(x₂,y₂) is therefore√{[Min {(1−|x₁−x₂|),|x₁−x₂|}]²+[Min {(1−|y₁−y₂|),|y₁−y₂|}]²}.

We also recall at this point that each node has a list 22 (FIG. 1) witha number of entries representing links to other nodes. Each entryconsists of the label and address of such another node. Initially thislist is empty and therefore the node has a second, similar, list ofbootstrap links—that is, a few links (typically four) so that it isinitially able to contact other nodes of the network. As well as thelinks in the list 22 (referred to as short-range links), the node mayalso have additional such lists arranged hierarchically, and/or a listof long-range links. These are described in our earlier internationalpatent application, but, as they are optional, are not described here.

Messages

Firstly, the following messages are used (note that the messages used inthe primary virtual network are different from, and completelyindependent of, the messages used in the secondary virtual network):

FIND messages are used to initiate and fulfil node look-ups and tosupport “PULL” updates. They contain:

-   -   the label of a target node    -   the address of the node that initiated the query        FOUND messages are used to return the results of queries. They        contain:    -   the label of the target node    -   the label of the node that was found    -   the address of the node that was found    -   the address of the node of the secondary network that is        associated with the node that was found    -   application-specific data—in this case the address of the        comment node that is associated with the node that was found        PUSH messages advertise a node's label to other nodes. They        contain:    -   the label of a subject node    -   the address of the subject node    -   the number of hops to go to reach a target node        NOTIFY messages are used to propagate push-updates. They        contain:    -   the label of a subject node    -   the address of the subject node        Retrieval

FIG. 11 shows how each node handles incoming Find messages. Inprinciple, the receiving node looks for a node which is closer thanitself to the target node identified in the Find message and, ifsuccessful, passes on the Find message. If not successful, it returnsits own address and label. It does this by carrying out the followingsteps:

-   Step 1100: the node receives a Find message which contains the label    of a target node and the address of an initiating node;-   Step 1105: the node translates the label of the target node into    co-ordinates in label space and calculates which, of all the links    (nodes) it has recorded is closest to the target node in label    space. The relevant node is designated nearest node;-   Step 1110: the node compares the distance between its own    co-ordinates and those of the target node with the distance between    the co-ordinates of nearest node and those of the target node;-   Step 1115: if the distance between its own co-ordinates and those of    the target node is less (or equal), the node sends to the initiating    node, via the network 10, a Found message containing its own label    and address;-   Step 1120: if the distance between the co-ordinates of nearest node    and those of the target node is less, the node forwards the Find    message to nearest node.    The address of the node returned in Step 1115 is either that of one    with the target label, or one close to it in label space. When the    returned label does not match the target label, it may mean either    that the target node does not exist or that the virtual network is    not sufficiently self-organised.    Push

Each node spontaneously initiates Push updates. For instance, each nodemight start a Push update process periodically. In a Push update, a nodesends out a Push message with its own label and address through a randomseries of nodes, setting a limit on the length of the series. The lastnode in the series sends a Notify message back towards the initiatingnode. FIGS. 12, 13 and 14 show the various parts of this process.

FIG. 12 shows how a node initiates a Push update, using the followingsteps:

-   Step 1200: the node selects a link randomly from amongst its    bootstrap links and enters the address of the node identified by the    selected link as a forward address for a next message;-   Step 1205: the node enters a small positive random number for the    field hops to go in the Push message;-   Step 1210: the node enters its own label and address as those of the    subject node in the Push message and sends the Push message to the    node at the forward address, using the network 10.

FIGS. 13 and 14 show how short range links are updated. Push messagesare used together with Notify messages to update short range links.There are two phases in this updating. In a first phase, each noderandomly forwards the Push message until the value in hops to go in themessage as received is “0”. If the value in hops to go is “0”, thereceiving node will start the second phase of the Push update by sendinga Notify message. In the second phase, the Notify message issuccessively forwarded to nodes whose labels are progressively closer tothe subject node's in the virtual space. If no node with a closer labelcan be found, then if necessary the links for the last found node areupdated. This is always the case when it would otherwise be unable tofind the given subject node, for instance because it had no short rangelinks yet established. The last found node then also sends additionalNotify messages to nodes that could potentially improve their link setsas well.

Referring to FIG. 13, the first phase of a Push update, dealing withincoming Push messages, involves the following steps:

-   Step 1300: a node receives a Push message. The Push message will    contain the label and address of an initiating node as the subject    node and will have a value in the field hops to go;-   Step 1305: the receiving node selects a link randomly from amongst    its bootstrap links and enters the address of the node identified by    the selected link as a forward address for a next message;-   Steps 1310 and 1315: the receiving node decreases the value in the    field hops to go by 1 and checks whether the decreased value for    hops to go is still greater than zero;-   Step 1320: if the decreased value is still greater than zero, the    node forwards the Push message to the forward address which it has    entered;-   Step 1325: if the value is zero, the node instead enters the label    and address of the initiating node (given in the received Push    message) as the subject node in a Notify message and sends the    Notify message to the forward address which it has entered.

Referring to FIG. 14, the second phase of dealing with Push updates,dealing with Notify messages, involves the following steps:

-   Step 1400: a node receives a Notify message containing the label and    address of a node as the subject node;-   Step 1401: the receiving node checks whether the subject of the    Notify message has the same label as the receiving node;-   Step 1402: if so, the receiving node checks whether the subject of    the Notify message has the same address as the receiving node. In    that case it takes no further action;

If however the subject of the Notify message is a node with the samelabel as, but an address different from, the receiving node, then twoevents occur. Firstly (Step 1403) the receiving node sends to thesubject node of the incoming Notify message a Notify message naming assubject a randomly-chosen node from the receiving node's own list ofshort-range links. Secondly, Step 1404 causes the generation of a Notifymessage for action by the secondary network. However, the receiving nodecannot generate such a message directly. In general we prefer to avoidsending, over the communication network, messages between differentvirtual networks, but the main problem is that the receiving node wouldneed not only the address of its own node of the secondary network, butalso the address of the node of the secondary node that is associatedwith the subject node. The receiving node does not have this address.Therefore, a two-stage process is used.

First, the receiving node sends a special CrossNotify message to thenode of the primary network specified as the subject in the incomingNotify message. This message contains:

-   -   a sender address, set to the address of the receiving node (i.e.        the node that received the incoming (primary network) message);    -   a receiver (or destination) address, set to the address        contained in the incoming Notify message;    -   a subject address, set to the address of the node of the        secondary network associated with the receiving node.

Note that the first two addresses are the addresses of nodes on theprimary network and the third address is the address of a node on thesecondary network.

Secondly, the node of the primary network that receives the CrossNotifymessage, in effect, forwards it to the associated node of the secondarynetwork. If necessary, the forwarding node could reformat the messageinto the format in use on the secondary network and replace the (primarynetwork) receiver address with the address of the associated node of thesecondary network. The message would then be handled just as shown inFIG. 3. The reason that we say “in effect” is that, in practice weprefer that the node of the primary network that receives theCrossNotify message just sends, to its associated node of the secondarynetwork, a simple, local message containing the address specified in thesubject field of the CrossNotify message. In that case the process ofFIG. 3 would be modified to include the step of setting notifylevel to asuitable value.

This process will be illustrated by means of an example, with referenceto FIG. 15 where the boxes represent nodes and arrows representmessages. Suppose a node P1 of the primary network receives, in step1400 of FIG. 14, a Notify message containing the label L_(P2) andaddress A_(P2) of the node P2 of the primary network as subject. At thenode P1 it is recognised (Steps 1401, 1402 in FIG. 14) that the subjectnode has the same label as P1 (i.e. L_(P1)=L_(P2)) but a differentaddress (A_(P1)≠A_(P2)). The node P1 knows the address A_(S1) of itssecondary network node S1, and generates (at Step 1404 in FIG. 14) aCrossNotify message with sender address A_(P1), receiver address A_(P2)and subject address A_(S1). This message is received at node P2 of theprimary network and this sends a local notify message, with the addressA_(S1), to the associated node S2 of the secondary network.Alternatively, the node S2 of the secondary network, upon receipt of theLocalNotify message, could, instead of creating the link itselfaccording to the process of FIG. 3, generate a further Notify message(of the secondary network) (shown by the dotted line in FIG. 12) whichit sends to the node S1, naming itself as subject. The Notify message isthen processed at node S1 which then uses the process of FIG. 3. Thisoption involves an additional message but has the advantage that, whenthe process of FIG. 3 comes to be executed, the Notify message hasactually been sent by the node whose address is in the subject field ofthe message, and the subject node has thus inherently been confirmed asstill being in existence.

Returning now to FIG. 14: Step 1405: the receiving node translates thelabel of the subject node into co-ordinates and calculates which of theshort range links it has recorded leads to a node label whoseco-ordinates are closest to those of the subject node in virtual space.The relevant node is designated nearest node;

-   Step 1415: the receiving node compares the distance between its own    co-ordinates and the co-ordinates for the subject node with the    distance between the co-ordinates for the nearest node and the    coordinates for the subject node.    If, at Step 1415, the distance between the receiving node and the    subject node is found to be the same or less, the receiving node    adds the label and address of the subject node as a link in its own    short range link set ((step 1420): this process is further discussed    below with reference to FIG. 16), sends to the subject node a Notify    message which contains the label and address of the receiving node    (step 1430) and sends to the nearest node a Notify message which    contains the label and address of the subject node (Step 1435);    If, at Step 1415, the distance between the nearest node and the    subject node is found to be greater, the receiving node reverts to    Step 1435 in that it sends to the nearest node a Notify message    which contains the label and address of the subject node.

FIG. 16 shows in detail how a node behaves when it updates itsshort-range links. It adds the new link to its short-range links andremoves all short-range links that are superseded by this link.

Referring to FIG. 16, a node may need to add a new link to its list ofshort range links, for instance as a result of Step 1420 in FIG. 14.

-   Step 1600: the updating node (that is, a node which is carrying out    an update to its short range link set) has the label and address of    a node for a new link;-   Step 1605: the updating node identifies all existing links which are    in respect of nodes which are closer to the new node than to the    updating node. These identified links are to be superseded. To    identify these links, the updating node calculates, for each    existing link, the distances between the co-ordinates for the new    node and the co-ordinates for the nodes specified in its existing    links. It compares each of these distances with the distance between    its own co-ordinates and the co-ordinates for the node specified in    the respective existing link;-   Step 1610: all links where the distance in relation to the new node    is less than the distance in relation to the updating node are    removed from the short range links;-   Step 1620: the updating node adds a link for the new node to its    short range links.

1. A computer network containing nodes, said network comprising: firstretrieval means responsive to input of a label to identify the addressof a node matching that label; and second retrieval means connected toreceive an address identified by the first retrieval means and operablein response thereto to identify the addresses of further nodes matchingthe same label; wherein each node matching a given label has associatedwith it a data storage area for containing the addresses of other nodesmatching the same label and is responsive to enquiry messages to returna message containing the addresses of the list; and wherein the secondretrieval means is operable to send an enquiry message to the addressidentified by the first retrieval means and upon receipt of a responseto iteratively send enquiry messages to addresses contained in theresponse to that enquiry message or as the case may be in a response toa subsequent enquiry message.
 2. A computer network according to claim 1in which the first retrieval means is formed by a primary network ofvirtual nodes, each node being defined by a list of links to other nodesof the secondary network, each entry in the list including a label andaddress of the respective other node; and wherein each node includesmeans responsive to receipt of a request message containing a label topropagate the request message within the network and means responsive toreceipt of a request message containing a label matching the label ofthe node receiving it to generate a reply message.
 3. A computer networkaccording to claim 2 in which the second retrieval means is formed by asecondary network of virtual nodes, each node being defined by a list oflinks to other nodes of the primary network, each entry in the listincluding an address of the respective other node; and wherein each nodeincludes means responsive to receipt of a request message to generate areply message containing the addresses of the list.
 4. A computernetwork according to claim 3 in which the reply message generated by anode of the primary network includes the address of that node of thesecondary network which is associated with the node generating the replymessage.
 5. A computer network according to claim 3, wherein: each nodeof the primary network includes means operable to initiate and topropagate exploratory messages each containing the label and address ofthe initiating node of the primary network; each node is operable uponreceipt of an exploratory message containing a label matching that ofthe receiving node and an address not matching that of the receivingnode to generate a notification message for addition of a link to thesecondary network, said notification message identifying the nodeinitiating the exploratory message and containing the address of thenode of the secondary network associated with the receiving node.
 6. Acomputer network according to claim 5, in which the notification messagecontains, as destination, the address of the initiating node, and theinitiating node is operable upon receipt thereof to forward to the nodeof the secondary network associated with the initiating node a messagerequesting addition of a link between it and the node having the addresscontained in the notification message.
 7. A computer network accordingto claim 1 in which each node of the secondary network includesprocessing means programmed to perform the following operations:receiving messages; responding to messages requesting information aboutthe contents of the list; complying with received requests to remove anaddress from the list and insertion of another address into the list;and in response to receipt of a message requesting a link between thenode and a second node: (A) generating a message to the second noderequesting information about the contents of its list; (B) determiningwhether both the first node and second node has in each case a number ofaddresses in its list which is less than the predetermined number; (C)in the event that this condition is satisfied, inserting into its listthe address of the second node and generating a message to the secondnode requesting the second node to add to its list the address of thenode; (D) in the event that this condition is not satisfied, determiningwhether the node has a number of addresses in its list which is at leasttwo less than the predetermined number, and if so (a) selecting from thelist of the second node the address of a third node; (b) inserting theaddress of the second node into the list of the first node and insertingthe address of the third node into the list of the first node; (c)generating a message to the second node requesting the removal of theaddress of the third node from the list of the second node and insertionof the address of the node; and (d) generating a message to the thirdnode requesting the removal of the address of the second node from thelist of the third node and insertion of the address of the node.
 8. Amethod of operating a computer network containing nodes, said methodcomprising: performing, in response to input of a label, a firstretrieval operation to identify the address of a node matching thatlabel; and performing, in response to an address identified by the firstretrieval operation, a second retrieval operation to identify theaddresses of further nodes matching the same label; wherein each nodematching a given label has associated with it a data storage area forcontaining the addresses of other nodes matching the same label and isresponsive to enquiry messages to return a message containing theaddresses of the list; and wherein the second retrieval operationincludes sending an enquiry message to the address identified by thefirst retrieval operation and upon receipt of a response iterativelysending enquiry messages to addresses contained in the response to thatenquiry message or as the case may be in a response to a subsequentenquiry message.
 9. A method according to claim 8 in which the firstretrieval operation uses a primary network of virtual nodes, each nodebeing defined by a list of links to other nodes of the primary network,each entry in the list including a label and address of the respectiveother node; and comprises: receiving a request message containing alabel; propagating the request message within the network; and uponarrival at a node of a request message containing a label matching thelabel of the node receiving it, generating a reply message.
 10. A methodaccording to claim 9 in which the second retrieval operation uses asecondary network of virtual nodes, each node being defined by a list oflinks to other nodes of the secondary network, each entry in the listincluding an address of the respective other node; and including, uponarrival of request message at a node, generating a reply messagecontaining the addresses of the list of that node.
 11. A methodaccording to claim 10 in which the reply message generated by a node ofthe primary network includes the address of that node of the secondarynetwork which is associated with the node generating the reply message.12. A method according to claim 10 , wherein each node of the primarynetwork includes means operable to initiate and to propagate exploratorymessages each containing the label and address of the initiating node ofthe primary network; and including, upon arrival at a node of anexploratory message containing a label matching that of the receivingnode and an address not matching that of the receiving node, generatinga notification message for addition of a link to the secondary network,said notification message identifying the node initiating theexploratory message and containing the address of the node of thesecondary network associated with the receiving node.
 13. A methodaccording to claim 12, in which the notification message contains, asdestination, the address of the initiating node, and the initiating nodeupon receipt thereof forwards to the node of the secondary networkassociated with the initiating node a message requesting addition of alink between it and the node having the address contained in thenotification message.
 14. A method according to claim 10 furthercomprising: (i) receiving a or the notification message requesting alink between a first node and a second node of the secondary virtualnetwork; (ii) determining whether both the first node and second nodehas in each case a number of addresses in its list which is less thanthe predetermined number; (iii) in the event that this condition issatisfied, inserting the address of the first node into the list of thesecond node and inserting the address of the second node into the listof the first node; (iv) in the event that this condition is notsatisfied, determining whether the first node has a number of addressesin its list which is at least two less than the predetermined number,and if so (a) selecting from the list of the second node the address ofa third node; (b) removing the address of the third node from the listof the second node; (c) removing the address of the second node from thelist of the third node; (d) inserting the address of the second nodeinto the list of the first node and inserting the address of the thirdnode into the list of the first node; and (e) inserting the address ofthe first node into the list of the second node and inserting theaddress of the first node into the list of the third node.
 15. A methodaccording to claim 14 in which the message requesting a link is receivedat the first node and in which, in step (iii): the address of the secondnode is inserted into the list of the first node accompanied by a markerindicating that it is unconfirmed; a message is sent from the first nodeto the second node requesting the second node to add the address of thefirst node to the links of the second node; at the second node theaddress is so added and an message of confirmation is sent to the firstnode; and at the first node upon receipt of the message ofacknowledgement the “unconfirmed”marker is removed.
 16. A methodaccording to claim 15 in which a node, upon receipt of a messagerequesting that it add to its list the address of a specified node,firstly sends a message to the specified node requesting a copy of thelist of the specified node, and then complies with said request only ifit receives from the specified node a list which contains the address ofthe node receiving the request.
 17. A method according to claim 14 inwhich the message requesting a link is received at the first node and inwhich the first node sends to the second node a request for a copy ofthe list of the second node; the second node sends the requested copy tothe first node; step (iv) (a) of selecting from the list the address ofa third node is performed at the first node; and steps (iv) (a) and (b)are performed in that: the first node adds the address of the secondnode and the address of the third node to the list of the first node, ineach case accompanied by a marker indicating that it is unconfirmed; thefirst node sends to the second node a message requesting that it removefrom its list the address of the third node and replace it with theaddress of the first node; the first node sends to the third node amessage requesting that it remove from its list the address of thesecond node and replace it with the address of the first node; thesecond node upon receipt of such message removes from its list theaddress of the third node, replaces it with the address of the firstnode and sends a message of confirmation to the first node; the thirdnode upon receipt of such message removes from its list the address ofthe second node, replaces it with the address of the first node andsends a message of confirmation to the first node; the first node uponreceipt of the message of confirmation from the second or third noderemoves the respective “unconfirmed” marker from its list.
 18. A methodaccording to claim 17 in which a node, upon receipt of a messagerequesting that it remove from its list the address of another node andreplace it with the address of a specified node, firstly sends a messageto the specified node requesting a copy of the list of the specifiednode, and then complies with said request only if it receives from thespecified node a list which contains the address of the node receivingthe request.
 19. A method according to claim 16 in which the listrequest message to the specified node, and a reply to such message, aresent via an intermediate node in such a manner that the address of thenode sending the list request message is not communicated to thespecified node.
 20. A method according to claim 14 in which each nodealso has means for storing at least one spare link, and including: inthe event of receipt of a message requesting a link between a first nodeand a second node when the first node has a number of addresses in itslist equal to the predetermined number, inserting the address of thesecond node into said spare link storage; and upon receipt of a latermessage requesting a link between the first node and another node,forwarding that message to the or an address retrieved from the sparelink storage means of the first node.